Abstract

The bacteriophage Mu mom gene encodes the unique DNA-modiÆcation function of the phage. Regulation of the mom gene at the transcriptional level is brought about by the transactivator protein C of the phage. The mom promoter is an activator-dependent weak promoter having poor 10 and 35 elements separated by a 19 bp suboptimal spacer region. These features could constrain RNA polymerase occupancy at the promoter.Here, we have probed into the mechanism by which C protein acts as a transcriptional activator at Pmom. In vivo dimethyl sulfate footprinting studies demonstrate C protein-mediated asymmetric distortion of its specific site at the mom regulatory region. Using a coupled topoisomerase assay, we demonstrate that C protein induces the unwinding of DNA.This C-mediated unwinding seems to be localised to the 30 Øanking region of the C binding site located adjacent to and overlapping the 35 element of Pmom. These results suggest that C protein-mediated torsional changes could be reorienting the ¡10 and ¡35 elements to a favorable conformation for RNA polymerase occupancy at the mom promoter.